In scientific research, as well as in other applications, it is often desired to illuminate an object of interest, such as a biological specimen, with filtered light. As a simple example, a specimen may be dyed with a florescent substance or marker, and illuminated by light of a particular wavelength to effectuate the desired florescence. The use of interference gratings that allow only transmission of a very narrow band of light wavelengths is a well known method of providing filtered light. Such a filter may sometimes be referred to as a bandpass filter. Typically, white light is filtered to produce the desired wavelength for illuminating the object under investigation. As used herein, the term “light” is intended to have broad meaning and to encompass not only visible light but also infrared and ultraviolet light subject to filtering. Further, as used herein, the term white light is intended to have broad meaning.
In many research applications, the object under investigation is very small, such that a microscope is necessary to conduct the desired experimentation. In such cases, and in others, space is at a premium and it is desirable to make the various hardware systems used to conduct the investigation as compact as possible. Such hardware systems may include, for example, optical systems for illuminating a specimen with filtered light, mechanical systems such as micromanipulators and the like to position the specimen, microscopy systems and probe systems for observing and making measurements of the specimen, recording systems for acquiring data and images, and control systems for operating and coordinating the hardware.
Filter wheels useful in such applications, and others, are well known. Basically, a filter wheel comprises a plurality of optical filters mounted on a disc-shaped “wheel” that is rotatable about a central axis. By rotating the wheel any of the filters can be positioned by the user in the light path, thereby allowing the user to select (from among the filters) the wavelength of light used to illuminate the specimen. Such filters wheels are available, for example, from Sutter Instrument Company of Novato, Calif., (www.sutter.com) assignee of the present invention.
It is known that the bandpass frequencies of an optical interference filter shifts as a function of the angle of incidence of the light directed onto the filter. Recently, interference filters have been developed taking advantage of this property over a broad range of angles without substantial loss of the desired bandpass properties. Hence, these newly developed filters are “tunable” over a substantial range of wavelengths by changing the angle of incident light. Such filters remain useful at angles of up to 60 degrees relative to the light path. Recently, the assignee of the present invention has developed a filter wheel incorporating such filters, wherein at least one, and preferable all, of the filters on the wheel are tunable. In the embodiment that has been developed, the filter wheel is rotatable about two axes, namely, a first axis that is perpendicular to the center of the wheel (as in past filter wheels), and a second axis that is in the same plane as the wheel disk and runs through the center of the disk. This second axis allows adjustment of the bandpass frequencies, in the manner described, by allowing the filter to be rotated relative to the incident light.
In many research applications, it is beneficial to be able to shine both filtered light and white light onto the object under investigation. For example, filtered light may be used, as described, to cause florescence at a specific frequency, and white light may be used to facilitate manipulation, preparation or handling of the object under investigation, or of hardware, such as a microelectrode, that is associated with the investigation. As noted above, however, the space used for the investigation may be very limited, and use of multiple light sources may be difficult or impossible due to space limitations. Likewise, in order to enable an experiment to proceed efficiently, it may be desirable to quickly switch between white and filtered light.